"At a high level, we're rolling out a social platform that will draw on the hundreds of millions of connections on Yahoo -- everything from random encounters with someone who commented on the same photo as you, to deep connections you have with friends who know nearly everything about you," said Jay Rossiter, head of Yahoo Open Strategy, in a blog post. "By using the social contacts you already have on Yahoo -- through Mail, Messenger, Flickr, Finance, Fantasy Sports, etc. -- we’ll make those social connections more active and useful. Most importantly, by enabling developers to make your social connections specific to the Yahoo! service you're using, we believe you’ll enjoy some incredibly unique and creative new experiences that we would never have thought of."

The release includes three major elements:

First, there's the Yahoo Social Platform, a set of REST-based social application programming interfaces (APIs) for utilizing social data related to Profiles, Connections, Updates, Contacts, and Status.

Second, there's Yahoo Query Language, a new Web service API for accessing other Web services using a SQL-style query language, rather than a lower-level programming code. Yahoo describes it as "a command-line version of Pipes," Yahoo's visual programming system for mashing up and remixing Web data, like RSS feeds.

Third, there's the Yahoo Application Platform, a set of software and services to build applications that run on Yahoo. It includes a browser-based development environment, various APIs and Web services, a distribution and discovery infrastructure, and a runtime and rendering environment.

Last week, at a Y!OS preview for the media, Ash Patel, head of Yahoo's audience product division, explained that Yahoo's Open Strategy "is about changing Yahoo from a walled garden to the best of the Web."

The week before that, Yahoo released its "universal profile," a reworked identity scheme for managing personal information and activities within Yahoo's rewired social infrastructure.

Over the coming months, Yahoo expects newly developed social applications to make the experience of using Yahoo more like Facebook or MySpace. Its goal is to drive more traffic to Yahoo and its partners and to encourage deeper engagement. If Yahoo is successful in doing so, it should generate more ad revenue as a result.

Yahoo aims to avoid some of the privacy pitfalls into which social networks like Facebook have stumbled. "We've done all of this in a way that keeps Yahoo as safe and secure as ever, while also building in full privacy and permission control so you'll have complete control over things like what you broadcast publicly and what information you share with third-party sites, etc.," said Rossiter.

Time will tell whether Yahoo can open up its infrastructure without opening potential vulnerabilities.